We can roughly divide the world in two regions: public space, where anyone can enter and watch what happens, and private space, where only selected people can get in and see what takes place inside. Of course, there are borderline conditions, such as space being either private or public depending on time of day (e.g., parks that close at night), some space being "public" but technically private property subjected to house regulations (e.g., inside a shop), and some space being "private" but with the technical possibility for outsiders to look inside (e.g., through windows, into a private home or vehicle, with convoluted mechanisms to explain why this is still a violation of privacy). Still, by and large, the distinction holds.
It seems that, by definition, there can be no expectation of privacy in public space, because, in principle, everything that happens there can be witnessed and recorded by anyone (and possibly shared publicly, although personality rights may apply). Yet, people rely on it to some extent. First, in terms of proximity: when you are with someone else (e.g., in a park) and there is no one around, you assume that your conversations are private, and (except in crowded environments) a third party cannot intrude and listen (because of the fuzzy assumption that, as long as they can sit elsewhere and enjoy an equivalent portion of the space, you should be entitled to your own spot; and that, when you talk and there is no one around, the information will not be recorded from far away). Second, in terms of unrelatedness, as people will sometimes indulge in a conversation with third parties within earshot under the assumption that they are not concerned by what is said (and politeness would require them not to pay attention). Third, in terms of continuity: you assume that no one knows your whereabouts at all times, because to do so they would have to stay close to you always, that is, follow you, and there is again an assumption that other's use of public space should not be "guided" by yours. Fourth, in terms of ephemerality: even if someone were to see or hear, you may assume that they will not retain a record of it forever, and that it is not possible to look up public space information about the past, because it was not archived.
My point is to focus on the last two privacy expectations, and show that they can break down when the notion of "public space" becomes altered by technology.
Today, a variety of entities (shops, police, etc.) have already installed CCTV cameras (within their private property or with applicable permits) to monitor public space. Cameras tend to become more and more widespread, so that more and more public space is filmed. The resulting trail of data is not eternal, for practical and legal reasons; but both limitations tend to disappear as time passes. So eventually we may assume that an uncoordinated bunch of actors will store traces which, together, could be used to reconstitute the entire history of everything visible in public space.
Now consider a second step which is currently starting to happen: CCTV cameras that upload their recordings to the cloud rather than storing them locally. This seems natural as more and more computing and storage is centralized in datacenters rather than on individual devices. Now, as I see no reason why cloud providers should not remain an oligopoly (or even become a monopoly), suddenly a growing proportion of the acquired data (in raw form) is available to a small number of actors. Incidentally, wiretapping ensures that various secret agencies also get access to the data.
Add a third step where the storage space, processing power, and algorithmic sophistication of the cloud providers go to infinity. Suddenly, all those actors have a different kind of access to public space, which is not limited by the notion of presence which intuitively applies for humans. They can know everything that happens everywhere, or happened at any point in time. I call this total public space access. This marks the collapse of the two privacy expectations I mentioned.
Of course, this has far-reaching consequences. Organizations with total public space access can know where everyone is located and the history of everywhere they went. This is problematic because of all the private information (love affairs, political organization, etc.) which is revealed by location information. (There are currently easier ways to retrieve this information, in a less precise manner, for people carrying mobile phones, but with CCTV opting out becomes much harder.) Note that this also implies you cannot privately go from point A to point B through public space, even if A and B are private... A tentative workaround would be to cover your face so that you are not recognizable, but this may be illegal, and does not suffice: people tend to usually return to their private dwellings, so that total access to public space is sufficient to establish a continuous trail for them, and thus identify them even if their appearances are indistinguishable.
Of course, this is not the only way in which unrestricted public space access challenges usual privacy expectations. Consider names on doorbells. To my knowledge, there is currently no database harvested from them that provides all addresses where a certain name appears, and people therefore do not consider that putting their real name on the doorbell divulges the information in that direction, from their name to the address. Yet this is all information available in public space, so I am not sure about the general legal framework that would prohibit the construction of a reverse database as I described.
The disappearance of privacy in public space is not necessarily a bad thing in itself: unrestricted public space access is a power, so it can be used for good, or for evil. It can be used to fight crime: while it cannot ensure that crime is altogether prevented, it ensures that crimes committed in the public space always leave a trace that can be investigated. Under the (non-obvious) assumption that this trace cannot be tampered with, it means that the objective truth of any claim about public space can be assessed. It implies that criminals can no longer run away (assuming interference powers from the police to extract criminals from a hideout in private space, and assuming that private space regions are not well-connected, as is the case in real life).
It is not clear that the provability of public space crime would make it impractical, because some criminals may not care if they will get caught; but assuming that it does, the benefits for society is not just the crimes that are no longer committed, it is much higher: it means that precautions to prevent the crimes are no longer needed (bikes, doors no longer need to be locked, stuff can be left in public space without risk), and also that some efficient rental schemes become practically applicable (if, e.g., there is no longer a risk that the rented good is not returned). Beyond crime, unrestricted access to public space gives opportunity for smarter decisions in terms of traffic, queues, shops being opened or closed, bus schedules, etc. Indeed, a lot of practical inefficiencies are the result of insufficient knowledge of public space, which (currently, and assuming that algorithms are not a problem) is usually caused by insufficient available data.
I have claimed that total public space access, under the assumptions that I outlined, will eventually become a technological possibility, and the default situation would be that a small number of organisations get it and the general public doesn't. What should be done about this?
A first option would be to legally prohibit total access to public space, or make it impossible. A good umbrella term (coined, to my knowledge, by Louis Jachiet) is that of indiscriminate data acquisition in public space. The rationale is that while people taking pictures, tourists filming monuments, etc., are acquiring information in a targeted manner, total public space access would result from CCTV, Google cars, and other technologies which perform such broad captures. Such acquisition should not necessarily be prohibited, but should become a target for regulation.
A second option would be to ensure that the resulting public space archive is available to everyone under the same terms. Indeed, much of the reasons why total public space access is scary is because of the asymmetry between those who have it and those who don't. It means that certain companies, secret services, can know anything about you (and could, e.g., prosecute you for any minor offense you commit), and yet protect themselves so that others do not know anything about them (especially, their wrongdoings would remain unpunished). Of course, organizations with more means will always stand a better chance of finding something to use against you, but society could try to ensure that citizens can at least access the data and organize to scavenge it.
In this second case, I am not sure about whether I think the resulting society would be a good one. The panopticon is usually thought of as a bad thing, but, in another way, the fact that you have non-total visibility and memory of public space seems to me like a bug that should be fixed, not a feature. I wonder what the best compromise is.